Conventionally, study has been conducted on a multi-mode fiber, and many methods have been proposed for solving a transmission capacity limitation caused by a mode group delay difference. One of the proposed methods is that the mode groups are separated for electrically equalizing the group delay difference (Non-Patent Document 1). After that, based on the fact that the mode propagation angle corresponds approximately to the mode order in a step-index fiber, angular division multiplexing was proposed (Non-Patent Document 2). In addition, in 2006, mode group diversity multiplexing was proposed in which almost the same concept is applied to a graded-index fiber (Non-Patent Document 3). In all fibers described in those documents, each eigen mode is not used as an independent transmission channel represented in the orthogonal function system but is used as a mode group for multiplexing/demultiplexing using propagation angle differences, with the result that the transmission capacity is smaller than that of a single-mode fiber. That is, the technologies described in those documents are not those for increasing the transmission capacity to the maximum.
Non-Patent Document 4 describes mode division multiplexing in which each of the modes of a multimode fiber is made to correspond to a transmission channel.
The technology described in Non-Patent Document is known as a technology for implementing a conventional uncoupled multi-core fiber of homogeneous cores using a photonic crystal fiber.
Non-Patent Document 6 discloses a technology for reducing the crosstalk quantity between two cores, which have different propagation constants, to a fixed value or lower.